A system is known from U.S. Pat. No. 3,242,305 comprising a plasma torch in which the electrode and the nozzle are cooled by a stream of liquid, such as water. In this torch, the electrode is movable relative to the nozzle and to the electric contact of the nozzle when the torch is at rest. When a voltage is supplied to the torch, the cooling liquid is put under pressure by a hydraulic mechanism which causes the compression of a spring and the separation of the electrode and the nozzle, thereby creating an electric arc and priming the plasma-producing gas injected into the torch when the torch is started up. In this torch, the cooling fluid circulates as long as said system carries current, independently of the supply of gas.
It is known from French Pat. No. 2,385,483 to achieve the striking of the arc between the electrode and the nozzle by short-circuiting the electrode and the nozzle, the electrode being screwed and put into contact with the nozzle and then unscrewed, the distance between the electrode and the nozzle being then adjusted to the desired value. The torch disclosed in this patent comprises a cooling system using a liquid circulating in the region of the electrode and the nozzle. This circulation of cooling fluid occurs when the system carries current.
A torch has been proposed in French Pat. No. 2,562,748 which comprises a structure particularly well-adapted to the striking of the arc by a short-circuit between the electrode and the nozzle. More details of this striking of the arc by short-circuiting may be had by referring to French Pat. No. 2,556,549. In this process, the electrode and the nozzle are mounted to be axially movable until a mutual contact occurs in opposition to the action of an elastic return means returning the electrode and the nozzle to a maximum mutual separation position corresponding to normal operation.
The nozzle is thus freely slidably mounted in the body of the torch so as to come into contact with the electrode when the torch is applied against a workpiece. By disengaging the torch, an arc is produced between the electrode and the nozzle whereby it is possible to strike and maintain an electric arc between these parts, said arc being transferred to the workpiece to be cut.
When the electric arc is broken, the plasma-producing gas continues to be injected in the torch so long as the system carries current. This permits the cooling of the torch after use.
The system is not very economical, since plasma-producing gas is also injected in the absence of an electric arc. Further, it is found to be particularly troublesome when striking an arc by short-circuiting, since it requires great force for achieving this. If the workpiece to be cut is thin and is in overhanging relation, the striking of the arc becomes very difficult.